Method of producing thermoelectric material

ABSTRACT

A thermoelectric material is provided. The material can be a grain boundary modified nanocomposite that has a plurality of bismuth antimony telluride matrix grains and a plurality of zinc oxide nanoparticles within the plurality of bismuth antimony telluride matrix grains. In addition, the material has zinc antimony modified grain boundaries between the plurality of bismuth antimony telluride matrix grains.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/117,286 filed May 27, 2011, the contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to materials having thermoelectric properties andto thermoelectric devices.

BACKGROUND OF THE INVENTION

A thermoelectric device can be used to obtain electrical energy from athermal gradient (for example, a thermoelectric generator using theSeebeck effect), or to generate a thermal gradient from electricalenergy (for example, a thermoelectric refrigerator using the Peltiereffect). The discussion below is directed to the Seebeck effect, but thegeneral concepts also apply to applications of the Peltier effect.

A typical thermoelectric device is built up from several unicouples,which are typically pairs of thermally conductive p-type (P) and n-type(N) semiconductors. These unicouples are connected electrically inseries and thermally in parallel. Theoretically, the maximum efficiencyof the conversion of heat energy to electrical energy is given by:

$\zeta_{\max} = {\frac{\left( {T_{H} - T_{C}} \right)}{T_{H}}\frac{\sqrt{1 + {ZT}_{ave}} - 1}{\sqrt{1 + {ZT}_{ave}} + {T_{C}/T_{H}}}}$

where T_(ave)=(T_(H)+T_(C))/2 is the average temperature of thermalgradient having a hot temperature (T_(H)) end and a cold temperature(T_(C)) end, and Z is a figure of merit, defined as Z=S²σ/κ. The figureof merit Z depends on the macroscopic transport parameters of thematerials, namely the Seebeck coefficient (S), electrical conductivity(σ), and thermal conductivity (κ). A large figure of merit is providedby a thermoelectric material having a large Seebeck coefficient, highelectrical conductivity, and low thermal conductivity.

The Seebeck coefficient is further defined as the ratio of theopen-circuit voltage to the temperature difference between the hot andcold junctions of a circuit exhibiting the Seebeck effect, orS=V/(T_(H)−T_(C)). Since Z varies with temperature, a usefuldimensionless figure of merit can be defined as ZT.

By the end of the 1950s, the best bulk thermoelectric materials werefound to be alloys of bismuth telluride and antimony, which gave a roomtemperature ZT˜1. Workers in the thermoelectric field have beenattempting to improve the figure of merit over the past 40 years withoutmuch success. Increasing ZT is difficult because the three parameters S,σ, and k are all related to the free carrier concentration and areusually not independent. For example, doping typically increases thesemiconductor's electrical conductivity, but decreases its Seebeckcoefficient and increases the thermal conductivity. Efforts to reducethe lattice thermal conductivity by alloying also reduce the electricalconductivity by providing an extra scattering mechanism.

Dresselhaus and coworkers at MIT theoretically demonstrated that quantumconfinement of electrons and phonons within nanowires of athermoelectric material can increase the value of ZT. 1-D nanowires inparticular could reach ZT≈2-5 if the nanowire diameter lies in the rangeof 5-10 nanometers. Certain structures have been investigated, forexample such as described in Heremans, J. P. et al., “ThermoelectricPower of Bismuth Nanocomposites”; Phys. Rev. Lett.; 2002, 88, 216801;Venkatasubramanian, R. et al., “Thin-film thermoelectric devices withhigh room temperature figures of merit”; Nature; 2001, 413, 597-602;Harman, T. C. et al., “Thermoelectric quantum dot superlattices withhigh ZT”; Electron. Mater.; 2000, 29, L1-L4; Rabin, O. et al.,“Anomalously high thermoelectric figure of merit in Bi_(1-x)Sb_(x)nanowires by carrier pocket alignment”; APL; 2001, 79, 81-83; andDresselhaus, M. S. et al., “Low-dimensional thermoelectric materials”;PSS; 1999, 41, 679-682. However, these approaches do not provide asimple approach to making large-scale, low-cost thermoelectric devices.Conventional semiconductor device fabrication methods are unsuitable formanufacturing bulk samples, and are often expensive.

In automobiles, about 70 percent of energy derived from fuel is lost towaste heat and engine cooling. Only a small proportion of energyprovided by fuel combustion is used, and a large amount of thermalenergy is thrown away. Recovery of waste thermal energy is a bigchallenge in automotive industries due to the increasing energy crisis.Thermoelectric conversion of thermal energy to electrical energy couldbe an effective way to obtain electrical energy from otherwise wastedheat production. However, direct thermal to electric conversion (DTEC)technology currently faces two major challenges: low conversionefficiency and insufficient power density. Hence, improved materials anddevices having high thermoelectric conversion efficiency are urgentlyrequired.

In response to the need for high thermoelectric conversion efficiencymaterials, Zhang et al. have investigated thermoelectric materialscomprising two or more components, at least one of which is athermoelectric material (U.S. Pat. No. 7,309,830). However, a giventhermoelectric material system can have a wide range of compositionsthat may, or may not, exhibit high ZT values, and as such, Banerjee etal. have developed a process for determining an optimum range ofcompositions for a nanocomposite thermoelectric material system (U.S.Pat. No. 7,734,428).

In addition to the above, other factors such as grain size and grainboundary properties have been postulated to affect the properties ofthermoelectric materials. However, as of yet no process has beendeveloped to determine if there is and/or which optimum range of suchfactors can provide a thermoelectric material with an improved ZT.Therefore, a process to model, calculate and/or determine an optimumrange of grain related properties in which a thermoelectric materialexhibits high ZT values would be desirable.

SUMMARY OF THE INVENTION

A thermoelectric material is provided. The material can be a grainboundary modified nanocomposite that has a plurality of bismuth antimonytelluride matrix grains and a plurality of zinc oxide nanoparticleswithin the plurality of bismuth antimony telluride matrix grains. Inaddition, the material has zinc antimony modified grain boundariesbetween the plurality of bismuth antimony telluride matrix grains.

In some instances, the grain boundary modified nanocomposite has anelectrical conductivity greater than 21,000 S/m and a thermalconductivity less than 0.6 at temperatures less than and equal to 200°C. In addition, the grain boundary modified nanocomposite can have afigure of merit ZT greater than 1.0 at temperatures up to 100° C. Inother instances, the grain boundary modified nanocomposite can have afigure of merit ZT greater than 0.6 at temperatures up to 200° C.

When compared to an equivalent nanocomposite material not having zincantimony modified grain boundaries, the grain boundary modifiednanocomposite can have an electrical conductivity at least 30% greaterthan an electrical conductivity of the equivalent nanocomposite, athermal conductivity at least 35% less than a thermal conductivity forthe equivalent nanocomposite, a charge carrier mobility at least 60%greater than for the equivalent nanocomposite, and/or a figure of meritZT at least 80% greater than the figure of merit for the equivalentnanocomposite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 d are schematic illustrations of: (a) the grain structure ofa prior art thermoelectric material; (b) a schematic illustration of athermoelectric material having altered grain boundaries according to anembodiment of the present invention; (d) a schematic illustration of aprior art nanocomposite thermoelectric material; and (d) a schematicillustration of a nanocomposite thermoelectric material having modifiedgrain boundaries according to an embodiment of the present invention;

FIG. 2 is a graph illustrating calculated electron mean free path,electron wavelength, and carrier percentage occupation as a function ofdimensionless electron energy;

FIG. 3 is a schematic illustration of a model for treatment of grainboundary barrier height according to an embodiment of the presentinvention;

FIG. 4 is a graph illustrating effect of grain boundary properties onelectrical conductivity;

FIG. 5 is a graph illustrating effect of grain size on lattice (Kl) andelectronic (Ke) thermal conductivity;

FIG. 6 is a graph illustrating effect of grain boundary properties ontotal thermal conductivity;

FIG. 7 is a graph illustrating effect of grain boundary properties onSeebeck coefficient;

FIG. 8 is a graph illustrating calculated normalized Seebeck coefficientas a function of electron energy;

FIG. 9 is a graph illustrating effect of grain boundary properties ondimensionless figure of merit (ZT);

FIG. 10 is a graph illustrating Seebeck coefficient for a bulk (Bulk)and nanocomposite (NC) thermoelectric material as a function of grainboundary barrier height;

FIG. 11 is a graph illustrating electrical conductivity and electronicthermal conductivity for a bulk (Bulk) and nanocomposite (NC)thermoelectric material as a function of grain boundary barrier height;

FIG. 12 is a graph illustrating ZT as a function of grain boundarybarrier height;

FIG. 13 is a series of graphs illustrating: (A) figure of merit ZTversus temperature for a bismuth antimony telluride matrix with zincoxide nanoparticles and zinc antimony modified grain boundaries(BAZT—filled square data points) and a bismuth antimony telluride matrixwith no zinc oxide nanoparticles or modified grain boundaries (BAT—opensquare data points); (B) electrical conductivity versus temperature forthe BAZT and BAT materials; (C) Seebeck coefficient versus temperaturefor the BAZT and BAT materials; (D) power factor versus temperature forthe BAZT and BAT materials; and (E) thermal conductivity versustemperature for the BAZT and BAT materials;

FIG. 14 is a transmission electron microscopy (TEM) image illustrating:(A) a bismuth antinomy telluride matrix with zinc antinomy modifiedgrain boundaries; and (B) zinc oxide nanoparticles within the bismuthantinomy telluride matrix; and

FIG. 15 is a graph illustrating the x-ray diffraction (XRD) results ofthe BATZ material.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses a process for determining an optimumrange of compositions for a thermoelectric material system, within whichthe material system may exhibit generally high figure of merit values.As such, the process has utility for improving the efficiency ofexperimental design and production of thermoelectric materials.

The process for determining an optimum range of compositions for athermoelectric material system considers a variety of relevant factors,parameters and the like in order to determine which material systemsshould be considered and/or which range of compositions should bestudied in more detail. A thermoelectric material exhibiting adimensionless high figure of merit (ZT) needs to possess a high Seebeckcoefficient (S) for high voltage generation, a low electricalresistivity (p) to minimize Ohmic losses and a low thermal conductivity(k) to minimize heat conduction.

The relationship between ZT, S, p, and k can be expressed as:

ZT=S ² T/kp  Eqn 1

and/or as:

$\begin{matrix}{{ZT} = \frac{S^{2}T}{\rho \left( {k_{el} + k_{ph}} \right)}} & {{Eqn}\mspace{14mu} 2}\end{matrix}$

where k_(el) and k_(ph) are the electronic and phonon contribution tothe overall thermal conductivity k.

Typically, S, p, and k are interdependent with an increase of theSeebeck coefficient resulting in an increase in electrical resistivity,whereas a decrease in the electrical resistivity results in an increaseof the thermal conductivity. At least one approach for obtaining highfigure of merit values has investigated the insertion of nanoparticleswithin a thermoelectric material (U.S. Pat. No. 7,309,830). Materialsusing this approach can result in phonons being scattered by thenanoparticles, thereby reducing the lattice thermal conductivity whileleaving the electrical resistivity and Seebeck coefficient for thethermoelectric host matrix unchanged.

Elemental substitutions, also known as atomic substitutions, inpotential thermoelectric materials have imperfections on the order of 1angstrom (Å). Thus alloying additions can result in the scattering ofshort-wavelength phonons much more effectively than mid- andlong-wavelength phonons. Therefore, mid- and long-wavelength phononsdominate the heat conduction in alloys and thermoelectric materials thathave been doped with other elements not originally within the startingmaterial. In the alternative, the inclusion of additions such asnanoparticles in the size range of phonon wavelengths introduces anotherscattering mechanism that affects mid- and/or long-wavelength phonons,thereby providing an opportunity to reduce the thermal conductivity ofsuch materials below the alloy limit. However, which nanoparticles withrespect to their composition, size and size distribution, and which hostmatrix the nanoparticles should be added to has heretofore been adifficult task to predict. In response to the difficulty in predictingsuccessful thermoelectric material systems, a process to perform justthis task has been developed by Banerjee et al. (U.S. Pat. No.7,734,428).

An embodiment of the current process includes determining a materialcomposition to be investigated for the thermoelectric material anddetermining a range of values for a grain related property that isobtainable for the material composition using state of the artmanufacturing techniques. Once the material composition and the range ofvalues for the grain related property have been determined, a pluralityof Seebeck coefficients for the material composition as a function ofthe range of values can be calculated. In addition, a plurality ofelectrical resistivity values and a plurality of thermal conductivityvalues for the material composition as a function of the range of valuesfor the grain related property can also be calculated.

It is appreciated that once the plurality of Seebeck coefficients,electrical resistivity values, and thermal conductivity values have beendetermined, a range of figure of merit values as a function thereof canbe calculated and a generally maximum range of figure of merit valuescan be determined, such values being a function of the range of valuesof the grain related property. Naturally, once the maximum range offigure of merit values has been determined, a thermoelectric materialhaving the determined material composition and the grain relatedproperty(ies) corresponding to the maximum range of figure of meritvalues is manufactured.

In the alternative to the above outlined embodiment, a plurality ofmaterial compositions can be investigated in this manner and a singlematerial composition or a limited range of compositions having apotential and desired ZT are manufactured.

The grain related property can include any grain related property knownto those skilled in the art, illustratively including grain size, grainboundary barrier height, and the like. For the purposes of the presentinvention, the term “grain size” is defined as the average mean diameterof grains within a thermoelectric material obtained through any methodand/or technique known to those skilled in the art. For example and forillustrative purposes only, a grain size can be determined by taking astatistical average of a plurality of grain diameters from ametallographic cross-section of the material with a single graindiameter obtained by averaging at least two linear and orthogonalmeasurements across a given grain.

Also for the purposes of the present invention, the term “grain boundarybarrier height” is defined as the energy potential of a grain boundarythat will scatter an electron having less energy than the potential andallow an electron having more energy than the potential to passtherethrough.

The material composition to be investigated can be a bulk thermoelectricmaterial composition, or in the alternative, a nanocompositethermoelectric material composition. It is appreciated that the term“bulk thermoelectric material” refers to a polycrystalline materialwithout the presence of second phase particles such as nanoparticles ofan insulating type material. In the alternative, the term “nanocompositethermoelectric material” refers to a bulk thermoelectric material havingsecond phase particles such as nanoparticle insulating materialinclusions, e.g. nanoparticle inclusions such as silicon oxide, zincoxide, and the like.

The range of values for grain size of the material composition to beinvestigated can be between 5 and 100 nanometers (nm) while the range ofvalues of grain boundary barrier height can be between 10 and 300milli-electron volts (meV). In addition, the grain size of themanufactured thermoelectric material can be obtained by consolidating aplurality of nanoparticles having a mean diameter that is less than orgenerally equal to the final grain size of the material. The grainboundary barrier height of the manufactured thermoelectric material canbe altered and/or obtained by doping of the material and/or altering asurface of the plurality of nanoparticles used to manufacture thethermoelectric material. In some instances, the surface of the pluralityof nanoparticles is altered by applying a coating thereon before thenanoparticles are consolidated to produce the thermoelectric material.

Not being bound by theory, it is appreciated that a grain boundary is aresult of and/or forms from a crystallographic misalignment betweenadjacent grains. In addition, the misalignment results in a residualelectric charge across the grain boundary which can produce anelectrostatic potential commonly referred to as an interfacial barrierand/or grain boundary barrier height which can be measured using ACimpedance. As a first approximation, the magnitude of this interfacialbarrier, also known as the grain boundary barrier height, can becalculated from the expression:

$\begin{matrix}{E_{b} = \frac{{eN}_{t}^{2}}{8\; ɛ\; N_{D}}} & {{Eqn}\mspace{14mu} 3}\end{matrix}$

where N_(t) is the number density of traps, ε is the permittivity andN_(D) is the doping concentration. The trap density is generally unknownand can vary widely, however assuming a generally high doping level andreported values for N_(t) in the range of 10⁻¹¹-10⁻¹³ cm⁻³, an E_(b) ofaround 25 meV can be calculated.

The process can provide a thermoelectric material as schematicallyillustrated in FIGS. 1 a and 1 d. In particular, FIG. 1 b illustrates abulk thermoelectric material 100′ having altered and/or engineered grainboundaries 110′ when compared to the grain boundaries 110 of thematerial 100 illustrated in FIG. 1 a. In addition, FIG. 1 d illustratesa nanocomposite thermoelectric material 200′ having altered and/orengineered grain boundaries 210′ in addition to nanoparticle inclusions205′ when compared to the material 200 having the nanoparticles 205 andgrain boundaries 210.

The grain size of the thermoelectric material 100 and/or 200 can bealtered and/or engineered, e.g. by using nanoparticles with a desiredaverage size to consolidate and manufacture the material. In addition,local electronic and thermal properties of the grain boundaries can bealtered and/or engineered by controlling the interfacial compositionbetween the grains, that is the interfacial composition of the grainboundaries. For example and for illustrative purposes only, a secondphase can be engineered to be present at the interface between thegrains such as Pb_(0.75)Sn_(0.25)Se coatings on Pb_(0.75)Sn_(0.25)Te;CoSb₃ coatings on La_(0.9)CoFe₃Sb₁₂; and alkali-metal salt coatings on(Bi_(0.2)Sb_(0.8))₂Te₃. In fact, results from CoSb₃/La_(0.9)CoFe₃Sb₁₂and coated (Bi_(0.2)Sb_(0.8))₂Te₃ materials have shown moderateimprovements in the figure of merit ranging from 15-30%.

In order to incorporate a grain related property into a modeling and/ormanufacturing process, the scattering behavior of electrons, holesand/or phonons within a material can be useful. Not being bound bytheory, a theoretical simulation can be based on the Boltzmann equationwith relaxation time approximation. For example, a modified Callawaymodel with respect to the lattice of a thermoelectric material can beincorporated with scattering of phonons through grain boundaries,defects, nanoparticles, and the like provided by Equation 4 below:

τ_(c) ⁻¹=τ_(B) ⁻¹+τ_(U) ⁻¹+τ_(N) ⁻¹+τ_(A) ⁻¹+τ_(D) ⁻¹  Eqn 4

where τ corresponds to scattering time and the subscripts B, U, N, A andD correspond to boundary, Umpklamp, normal, alloy, and nanoparticle,respectively, related scattering.

With respect to carriers, that is electrons and holes, Equation 5 can beused where Op, DOp, and DAp represent optical phonon, deformationpotential of optical phonon, and deformation potential of acousticphonon related scattering.

τ_(ξ) ⁻¹=τ_(Op) ⁻¹+τ_(DOp) ⁻¹+τ_(DAp) ⁻¹  Eqn 5

In addition to scattering time, the total electrical conductivity can beexpressed as a summation of the contributions from both electron andhole bands, while the overall Seebeck coefficient can be obtainedthrough weighting each band's contribution using a normalized electricalconductivity. In order to obtain the electronic thermal conductivity,the electronic thermal conductivity from the Lorentz number (L) can beobtained using Equations 6-8 below. In particular, Equation 6 is anexpression of the total electrical conductivity (σ), Equation 7 is anexpression of the overall Seebeck coefficient, and Equation 8 is anexpression for the electronic thermal conductivity. It is appreciatedthat the bipolar thermal conductivity contribution to the electronicthermal conductivity must also be considered and that this type ofconduction occurs when carriers moving between different bands carryheat via the Peltier effect and as such can still transport heat even ifthe net electric current is zero.

$\begin{matrix}{\sigma = {\sum\limits_{i}^{e,h}\; \sigma_{i}}} & {{Eqn}\mspace{14mu} 6} \\{S = {\sum\limits_{i}^{e,h}\frac{S_{i}\sigma_{i}}{\sigma}}} & {{Eqn}\mspace{14mu} 7} \\{k_{e} = {\left( \frac{k_{B}}{e} \right)^{2}\left( {{\sum\limits_{i}^{e,h}L_{i}} + L_{b}} \right)\sigma \; T}} & {{Eqn}\mspace{14mu} 8}\end{matrix}$

In addition to the above, the nature of grain boundary scatteringexhibited by carriers can be estimated from the electron wavelength andelectron mean free path (MFP) and the cumulative distribution functionof the electron occupation number versus electron energy can provide thepercentage of electrons that have energy less than a certain value. Inparticular, Equations 9-13 afford for the electron MFP, electronwavelength, and carrier percentage occupation as a function ofdimensionless electron energy shown in FIG. 2 where the electron MFP canbe calculated using the expression 1=ντ where ν and τ are provided byEquations 11 and 4, respectively, g is the density of state function andα is equal to the inverse of the bandgap for the material (1/E_(g)), insome instances referred to as the parabolicity factor.

$\begin{matrix}{g = \frac{\int_{0}^{{E/k_{b}}T}{{g(E)}\ {E}}}{\int_{0}^{\infty}{{g(E)}\ {E}}}} & {{Eqn}\mspace{14mu} 9} \\{{g(E)} = {v^{2}{D(E)}\left( {- \frac{\partial f}{\partial E}} \right)}} & {{Eqn}\mspace{14mu} 10} \\{v = \frac{\left( {2{{E\left( {1 + {\alpha \; E}} \right)}/m_{c}^{*}}} \right)^{0.5}}{\left( {1 + {2\alpha \; E}} \right)}} & {{Eqn}\mspace{14mu} 11} \\{{D(E)} = {\frac{\sqrt{2}\left( m_{d}^{*} \right)^{1.5}}{\pi^{2}\hslash^{3}}{E\left( {1 + {2\alpha \; E}} \right)}\left( {1 + {\alpha \; E}} \right)}} & {{Eqn}\mspace{14mu} 12} \\{\lambda = {\frac{2{\pi\hslash}}{m_{c}^{*}v} = \frac{2{{\pi\hslash}\left( {1 + {2\alpha \; E}} \right)}}{\sqrt{2m_{c}^{*}{E\left( {1 + {\alpha \; E}} \right)}}}}} & {{Eqn}\mspace{14mu} 13}\end{matrix}$

As shown in FIG. 2, the majority of electrons have a MFP less than 30nanometers which is comparable and/or of the same order of magnitude ofa grain size of between 20 to a few hundred nanometers. As such, FIG. 2confirms that a majority of carriers will experience multiple scatteringdue to the grain boundaries and “memory” of a previous collision by acarrier will be retained by the carrier when it reaches another portionof the grain interface, i.e. each scattering point on a grain boundaryis not independent from others. In addition, since the dominant electronwavelength is approximately 28 nm and an assumed grain boundary width of1 to 2 nm is much smaller, there is no possibility of diffuse scatteringof electrons. Finally, the electron MFP of less than 30 nanometers iscomparable to a hole wavelength of 28 nm which implies that theBoltzmann equation is at the edge of its validity using this process. Assuch, it is appreciated that other expressions with respect torelaxation time approximation can be used for the basis of a theoreticalsimulation that incorporates grain boundary effect in determiningvarious properties of a thermoelectric material and still fall withinthe scope of the present invention.

Turning now to the actual effect of grain boundary properties onthermoelectric characteristics, FIG. 3 provides a model of a grainhaving a grain boundary with a width w and a grain boundary potentialbarrier of E_(b). In addition, the grain size has a dimension of L whichnaturally separates the grain boundary on opposing sides of the grain.As illustrated in the figure, if an electron has an energy of E, theelectron will pass through the grain boundary barrier height if E>E_(b)and will scatter if E<E_(b).

Not being bound by theory, assuming T(E) is a transmission probabilityof an electron passing through a grain boundary barrier height and thereare N grain boundaries, the MFP of the electron due to scattering by thegrain boundary can be expressed as Equation 14 when N is assumed to beinfinity.

$\begin{matrix}{\lambda_{grainboundary} = {{\sum\limits_{n = 1}^{N\rightarrow\infty}\; {{T(E)}^{n}\left( {1 - {T(E)}} \right){nL}}} = \frac{{T(E)}L}{1 - {T(E)}}}} & {{Eqn}\mspace{14mu} 14}\end{matrix}$

which further provides a relaxation time of:

τ_(B)=λ_(grainboundary)/ν  Eqn 15

where ν is given by:

$\begin{matrix}{\tau_{B} = {\frac{L}{v}\left( {1 + \frac{4\frac{E}{E_{b}}{{1 - \frac{E}{E_{B}}}}}{\sinh^{2}\sqrt{{{{\frac{2\; m_{c}^{*}E_{B}w^{2}}{\hslash^{2}}}1} - \frac{E}{E_{B}}}}}} \right)}} & {{Eqn}\mspace{14mu} 16}\end{matrix}$

In order to better understand the effect of grain related properties onthe thermoelectric material behavior, and based on the model shown inFIG. 3, the effect of grain size on electrical conductivity was examinedwith results shown in FIG. 4. The width w of the grain boundaries wasassumed to be constant at 2 nm while the grain boundary barrier heightwas varied from 20 meV to 300 meV. In addition, the electricalconductivity of a bulk thermoelectric material and a nanocompositethermoelectric material was investigated and is shown in the graph. Inthe case of the nanocomposite thermoelectric material, SiO₂nanoparticles of 3 nm diameter were used for the calculations. It isappreciated that FIG. 4 illustrates that with increasing grain size, theelectrical conductivity increases, which can be explained due todecreasing probability of scattering events. In addition, with theinclusion of ceramic nanoparticles within the material, significantlylower electrical conductivities were observed. Finally, varying thegrain boundary barrier height significantly affects the conductivitiesof both the bulk thermoelectric material and the nanocompositethermoelectric material. It is appreciated that this effect is strongerat smaller grain sizes simply due to the fact that smaller grainsincrease the number of scattering events and thus reduce a carrier MFP.

Turning now to FIG. 5, a graph illustrating the lattice and electronicthermal conductivity as a function of grain size is shown. Similar toFIG. 4, grain boundary scattering clearly affects both lattice andelectronic thermal conductivity with the most significant effectoccurring for grain sizes below 25 nm. In addition, FIG. 6 illustratesthat total thermal conductivity illustrates a similar behavior toelectrical conductivity which provides evidence that reduction in grainsize for a thermoelectric material can be an effective way of reducingthe material's thermal conductivity.

Regarding the Seebeck coefficient for a thermoelectric material, FIG. 7illustrates a complicated relationship between the Seebeck coefficient,grain size, and grain boundary barrier height. In particular, and forbulk thermoelectric material, the highest Seebeck coefficient occurredfor a grain boundary barrier height of 60 meV while for a nanocompositethermoelectric material, the highest Seebeck coefficient was observedfor a grain boundary barrier height of 20 meV. It is appreciated thatthe difference between the two materials and the associated Seebeckcoefficient can be the result of filtering of low energy electronswithin the grains of the nanocomposite thermoelectric material. Inaddition, FIG. 8 provides a typical normalized Seebeck coefficientdistribution as a function of electron energy. As shown by this figure,a maximum value or maximum range of values for the Seebeck coefficientdoes not result from electron energies that are too low or too high.Stated differently, there is an intermediate value or range of valuesfor electron energy that provides a desired Seebeck coefficient. Inaddition, low energy electrons pose a negative impact to the Seebeckcoefficient.

Based on these figures and their teachings, it is clear that smallgrains with high grain boundary barrier potentials, for exampleE_(b)=300 meV, have the least effect on the Seebeck coefficient sincesuch high potential barriers can filter even high energy electrons. Onthe other hand, FIG. 7 illustrates that the Seebeck coefficient behaviorflipped or was inverted for the nanocomposite material versus the bulkmaterial when the grain boundary barrier height was 20 meV and 60 meV,respectively. Not being bound by theory, this is postulated to be due tothe Seebeck coefficient distribution being different for the twomaterials, and depending on the location of the peak of normalizedSeebeck coefficient as shown in FIG. 8, either 20 meV or 60 meV can bemore effective in increasing the Seebeck effect.

Regarding the dimensionless figure of merit ZT, FIG. 9 provides acomparison of ZT as a function of grain size, bulk thermoelectricmaterial, nanocomposite thermoelectric material, and grain boundarybarrier height. From this figure, it is appreciated that grain sizesbelow 25 nanometers can provide a dramatic improvement in theperformance of bulk thermoelectric material and nanocompositethermoelectric material. In addition, the grain boundary barrier heightcan significantly alter the ZT for a particular material. For example,at lower grain sizes, the ZT for the bulk and nanocomposite materialsoverlap, thereby suggesting that the benefit of adding second phasenanoparticles to a thermoelectric material can be diminished in caseswhere the grain boundary barrier potential is high due to impurities,doping, and the like.

The effect of ceramic nanoparticle inclusions within a bulkthermoelectric material on grain boundary barrier height can also be ofinterest with FIG. 10 illustrating a graphical representation of Seebeckcoefficient as a function of grain boundary barrier height for a bulkthermoelectric material (Bulk) and a nanocomposite thermoelectricmaterial (NC). As shown in FIG. 10, and for which a grain size of 30 nmwas assumed, smaller grain boundary barrier heights are preferred withpotentials over 100 meV virtually having no effect on the Seebeckcoefficient of the material. In addition, FIG. 11 shows or illustratesthe same behavior with respect to electrical conductivity and electronicthermal conductivity and FIG. 12 provides a graph illustrating theeffect of grain boundary barrier height on ZT for bulk thermoelectricmaterial and nanocomposite thermoelectric material.

It is appreciated that FIG. 12 could lead to the conclusion that lowergrain boundary barrier heights are desired in all cases in order toachieve an increase in ZT for any thermoelectric material. However, sucha conclusion can be false, for example when the grain size is alsoconsidered as discussed above in relation to FIG. 9.

It is appreciated that the process of calculating the thermalconductivity and electrical resistivity for a given nanocompositematerial system as a function of material compositions affords for thecalculation of figure of merit values as a function of the compositions.In this manner, researchers can estimate which matrix host-nanoparticlesystems are more likely to exhibit relatively high ZT values and/orwhich compositions or range of compositions within a particular systemmay provide the highest ZT values. This range of compositions with theassociated high ZT values can also be compared with other materialproperties such as mechanical property data, chemical property data andthe like, in order to choose an optimum thermoelectric materialcomposition for a given application. As such, the process provides avaluable tool to guide experimental design of thermoelectric materials.

As disclosed above, the plurality of material positions to beinvestigated can include a first component with a volume fraction of asecond component ranging from 0.0 to 1.0. In some instances, thematerial compositions to be investigated can include the first componentwith a volume fraction of the second component ranging from 0.0 to 0.7.The plurality of thermal conductivity values are calculated as afunction of the scattering cross section of the second componentnanoparticles for the plurality of material compositions beinginvestigated. In addition, the scattering cross section can be afunction of the interfacial surface area of the second componentnanoparticles for the plurality of material compositions beinginvestigated. The function of the plurality of material compositionsbeing investigated can include the size of the second componentnanoparticles, the size distribution of the second componentnanoparticles and an interfacial property of the second componentnanoparticles. In some instances, an interfacial interaction propertybetween the second component nanoparticles and the first component canbe used.

It is appreciated that the thermoelectric device can be designed anddeveloped using the process disclosed herein, the thermoelectric devicehaving a first electrical contact, a second electrical contact, and athermoelectric bulk material located within an electrical path betweenthe first electrical contact and the second electrical contact. Thethermoelectric bulk material can include a first powdered componenthaving a particulate form, the first powdered component beingelectrically conducting, and a second powdered component having aparticulate form, the second powdered component having an electricalconductivity substantially less than the first powdered component. Thefirst and second powdered components can retain the particulate form inthe bulk thermoelectric material and the thermoelectric bulk materialcan be a composite that has nanostructures of the first powderedcomponent. The first component can be a metal or a semiconductor. Thesecond component can be an electrical insulator in the form of aceramic. It is appreciated that the process can also be used forsemiconductor-metal and semiconductor-semiconductor thermoelectricmaterial systems.

It is further appreciated that the bulk thermoelectric material can bean electrically conducting material such as a semiconductor or metal. Inaddition, the electrically conducting material can be an organicmaterial, or an organic material such as an organic semiconductor.

In the temperature range between 300K to 500K, an n-type material suchas Bi₂Te₃ or Bi₂Se₃ and/or the p-type material such as Bi₂Te₃ or Sb₂Te₃can be used for the bulk thermoelectric material. For the temperaturerange between 500K to 700K, n-type materials such as PbTe or SnTe dopedwith Bi and/or p-type materials such as PbTe or SnTe can be used. Inaddition, materials such as ZnSb, SiGe, CoSb, CeFeCoSb, and alloysthereof can be used for the bulk thermoelectric material. Regardingnanocomposite thermoelectric materials, nanoparticles of insulatingmaterials such as SiO₂, ZnO, Al₂O₃, LaCoO₄, NaCoO₄, SnO₂,(ZnO)_(x)(In₂O₅)_(y), ZrO, Y-stabilized ZrO, ZrO₂, yttria stabilizedZrO₂ (YSZ), La₂O₃ stabilized YSZ, other oxide materials, carbonnanoparticles, electrically insulating polymer nanoparticles, fullerenessuch as C₆₀.

The next step in the methodology advancement is to utilize phononscattering via inclusion of nanoparticles into a thermoelectric matrixto reduce phonon thermal conductivity, and also hybridize the matrixwith grain boundary modification to improve the carrier mobility, andtherein the power factor. To demonstrate the advantage of this dualapproach, a unique nanocomposite (referred to as BATZ) was created of abismuth antimony telluride matrix with both zinc antimony grain boundarymodifications and inter-grain phonon scattering zinc oxidenanoparticles. The power factor augmentation, in conjunction withreduction of thermal conductivity, resulted in an 83% improvement to thefigure of merit ZT compared to an analogous or equivalent sample withoutzinc-nanostructures (referred to as BAT). It is appreciated thatmodification of a thermoelectric matrix with two types of nano-features(one for scattering phonons and another to increase charge carriermobility) represents an important advancement in the generalthermoelectric structural methodology, and can be applied to othercombinations of thermoelectric materials and nanoparticles beyond thespecific examples disclosed herein. In addition, it is appreciated thatfor the purposes of the present invention, the term “analogous” and“analogous material” refers to a material having generally the samenon-oxide matrix composition and crystallite or grain size as themodified nanocomposite disclosed and discussed below and in FIGS. 13-15.

The BATZ material was made by means of a wet-chemistry synthesis thatfirst yielded an admixture of bismuth antimony telluride nanoparticlesand zinc oxide nanoparticles. This nanoparticle mixture was thenconsolidated, by hot pressing, to form a BATZ nanocomposite. Asindicated below, the BAT nanocomposite was formed in an analogousmanner, excluding the presence of zinc oxide nanoparticles and therebyprecluding the formation of complex zinc-nanostructures responsible forimproving the ZT from 0.6 to 1.1 (at 100° C.) as shown in FIG. 13A.

Nanoparticle Synthesis:

The synthesis of the BAT and BATZ nanoparticles was conducted asfollows. A reagent solution of sodium telluride hydride was made in thefollowing manner. Water (103 mL) and tellurium powder (5.91 g) wereadded to a flask degassed with inert gas, rapidly stirred and thencooled in an ice water bath. Sodium borohydride (6.32 g) was then added,in portions, and the reaction was allowed to stir for at least 12 hoursuntil all of the tellurium powder has dissolved. The product solutionwas filtered through a fritted glass filter, still excluding oxygen, tocollect a merlot-colored filter cake product solution. The filter cakewas then washed with water (15 mL), through the fritted glass filter,and combined with an initially collected quantity of sodium telluriumhydride solution.

A solution of water and 28% ammonium hydroxide (6.5 mL and 5.5 mL,respectively) was prepared, and a combination of potassium antimonytartrate (9.02 g) and bismuth citrate (1.54 g) were dissolved completelyin the diluted ammonium hydroxide solution. The antimony and bismuthsalts were dissolved in portions; rigorously dissolving each portionbefore adding more of the salts. The freshly prepared aqueous solutionof antimony and bismuth salts was then added to a reaction flask thathad previously been degassed with inert gas and charged with water (480mL). For the BATZ synthesis, a finely dispersed aqueous suspension ofzinc oxide nanoparticles was added to the reaction solution (2.27 g ofzinc oxide nanoparticles in 68 mL water).

The collected sodium tellurium hydride solution was then added dropwiseto the rapidly stirring reaction solution containing the dissolvedbismuth and antimony salts and zinc oxide nanoparticle suspension. Afteraddition of the sodium tellurium hydride solution was complete, thereaction was allowed to stir for an additional 20 minutes. The productwas then collected using centrifugation and washed under an inertatmosphere in a Soxhlet apparatus with a solution of water, methanol,and 28% ammonium hydroxide (35/165/0.8 respectively by volume). A finalrinsing with methanol was administered and the methanol-slurry ofnanoparticle product was dried under an inert gas flow and then groundto a fine powder, in a glovebox.

Nanocomposite Sintering:

Sintering of the composite nanoparticle powders was performed usinggraphite punch and dies and a hot press. All samples were first baked at400° C. for 20 minutes and then sintered at 400° C. and 100 MPa for 4hours under an argon atmosphere.

Temperature dependent transport properties for the BAT and BATZnanocomposites, between room temperature and 200° C., are shown in FIG.13A-13E. The electrical conductivity of the BATZ sample is consistentlyhigher over the entire temperature range measured as illustrated in FIG.13B. For example, at 150° C., the BATZ material exhibited a 34% higherelectrical conductivity versus the analogous BAT nanocomposite. HallEffect measurements were conducted to probe the electrical conductivityimprovement and the carrier concentration of the BAT controlnanocomposite was insubstantially 6% greater than the quantity of chargecarriers in the BATZ nanocomposite. However, the BATZ nanocompositecharge carrier mobility was found to be 67% higher than that of the BATmaterial and thereby was determined to dominate and be responsible forthe measured increase in electrical conductivity. It is appreciated thatthe material property comparisons are consistent with the aforementionedzinc antimony grain boundary modification of the BATZ nanocomposite andit is fundamentally different than previous reports of grain boundarymodification that relied on alkali metal salts or composites made fromelemental chunks. It is also appreciated that zinc oxide is n-type,which means there may be an injection of minority carriers in the systemdue to the zinc oxide inclusions, but the BATZ nanocomposite isundoubtedly p-type.

The Seebeck coefficient was over 200 μV/K at temperatures below 150° C.for both BATZ and BAT as illustrated in FIG. 13C. The Seebeckcoefficient of BATZ was lower than that of BAT below 150° C., but thentended to be higher above 150° C. Not being bound by theory, the smallerSeebeck coefficient at lower temperature is attributed to a highercarrier concentration in the heavily doped material, while the largerSeebeck coefficient of BATZ at higher temperature indicates suppressionof minority carriers (electrons). As a result of better electricalproperties, the BATZ material showed an overall improvement in thepower-factor throughout the measured temperature range as illustrated inFIG. 13D.

The BATZ material thermal conductivity ranged from 0.4 to 0.6 W/mK. Amaximum reduction of 41% in thermal conductivity (at 150° C.) wasrealized from adding zinc oxide nanoparticles as illustrated in FIG.13E. The correlation of a stronger reduction in thermal conductivity athigher temperatures is consistent with the electrical measurement data.In addition, and even though higher electronic thermal conductivity isexpected for the BATZ sample, the suppression of minority carrierscontributed to the reduction of bipolar thermal conduction, which causedthe overall dominance of phonon-driven thermal conductivity attemperatures higher than 150° C. Therefore, the inclusion of the zincoxide nanoparticles was an effective means of phonon scattering, in sucha thermoelectric system.

Transmission electron microscopy (TEM) imaging was conducted tocorrelate the structural origin of the electrical conductivity increasein the presence of a reduced thermal conductivity in the BATZ materialwhile maintaining a generally constant Seebeck coefficient when comparedto the BAT material. Zinc antimony formed from a reaction of thenanocomposite constituents during the sintering process, andprecipitated at the boundaries between bismuth antimony telluride grainsin the BATZ material as illustrated in FIG. 14A. Compositiondetermination was conducted with TEM-EDS and Z-contrast TEM. The zincantimony phase identification was further supported by itsidentification in the XRD spectrum as shown in FIG. 15. Zinc antimony isan inter-metallic semiconductor and also a thermoelectric material. Bothpolycrystalline and thin films morphologies of zinc antimony show anelectrical conductivity on the order of 40,000 S/m at room temperature,about 50% than that of the BATZ sample. Again, not being bound bytheory, formation of zinc antimony at the host-host grains is expectedto reduce the barrier potential and consequently reduce hole scattering.By reducing the interfacial electrical resistance in the grain, itresults in a higher electrical conductivity and consequently increasesthe power factor. The presence of zinc antimony at the grain boundarydoes not pose a thermal resistance as its thermal conductivity is1.5-2.5 W/mK at room temperature, which is higher than the BATZnanocomposite. Based on the physical size of the formed zinc antimony,it is not expected to contribute to the phonon scattering phenomenoneither, with an average width of 44±17 nm and length over 100 nm asillustrated in FIG. 14A.

Two other crystalline phases were identified in the XRD spectrum shownin FIG. 15, with bismuth telluride and antimony oxide present in boththe BAT and BATZ nanocomposites. Comparison of peak widths, for thenormalized XRD spectra, showed that the average crystallite sizes werequite similar between the two different materials. In addition, thebismuth antimony telluride lattice of the BATZ nanocomposite wasslightly expanded with the inclusion of zinc-nanostructures. Suchalternations to the lattice of the nanocomposite matrix were commonlyobserved throughout the hot-press sintered nanocomposites and theantimony oxide is believed to have formed while processing nanoparticlepowders by brief contact with air. Based on reference intensity ratioanalysis of the XRD spectra, the quantities of bismuth antimonytelluride oxidized to form antimony oxide for BAT and BATZ materialswere within 3% of each other, and therefore presumed to have anequivalent influence on the properties in both sintered materials.

Zinc oxide nanoparticles were visible throughout the bismuth antimonytelluride grains via TEM as shown in FIG. 14B. Characterization was,again, based on TEM-EDS and Z-contrast TEM, and the average diameter wasmeasured to be 10±4 nm. Neither aggregation nor anomalous growth of thezinc oxide nanoparticles was observed. As expected based on the size ofthe zinc oxide nanoparticles, there was no indication of zinc oxide inthe XRD spectrum of the BATZ nanocomposite and the absence of peaks inthe XRD spectrum is an affirmation of consistent nanometer-dimensionsfor the occurrences of zinc oxide throughout the nanocomposite. Antimonyoxide nanoparticles ranging 12 to 18 nm in diameter (in addition to theaforementioned large grains of antimony oxide reported by the XRDanalysis) were also observed by TEM in the BATZ nanocomposite. Theseantimony oxide nanoparticles occurred too rarely to count for ameaningful average diameter calculation. In addition, and based on theirscarcity, it is thought that the antinomy oxide nanoparticles do notappreciably influence the thermoelectric properties of thenanocomposite. Such antimony oxide nanoparticles were not observed byTEM in the BAT compaction and it is not believed that zinc oxidefacilitates antimony oxide nanoparticle formation, but that antimonyoxide nanoparticles were not observed with TEM due to their rarity.

The thermoelectric properties of the BATZ nanocomposite described weredisentangled by the addition of phonon scattering zinc oxidenanoparticles and the formation of charge carrier mobility-enhancingzinc antimony grain boundaries. This effective decoupling of theelectrical conductivity, Seebeck coefficient, and thermal conductivity,as shown here on multi-gram scale, is critical for the advancement ofthe field and its commercial viability. And in general, these twoapproaches to improving the ZT value, when combined in a singlenanocomposite, offer a new hybrid methodology in thermoelectric materialresearch.

The invention is not restricted to the illustrative examples describedabove. The examples are not intended as limitations on the scope of theinvention. Methods, apparatus, compositions and the like describedherein are exemplary and not intended as limitations on the scope of theinvention. Changes therein and other uses will occur to those skilled inthe art. The scope of the invention is defined by the scope of theclaims.

We claim:
 1. A thermoelectric material comprising: a grain boundarymodified nanocomposite having a plurality of bismuth antimony telluridematrix grains; a plurality of zinc oxide nanoparticles within saidplurality of bismuth antimony telluride matrix grains; and zinc antimonymodified grain boundaries between said plurality of bismuth antimonytelluride matrix grains.
 2. The thermoelectric material of claim 1,wherein said grain boundary modified nanocomposite has an electricalconductivity greater than 21,000 S/m and a thermal conductivity lessthan 0.6 at temperatures less than and equal to 200° C.
 3. Thethermoelectric material of claim 2, wherein said grain boundary modifiednanocomposite has a figure of merit ZT greater than 1.0 at temperaturesup to 100° C.
 4. The thermoelectric material of claim 3, wherein saidgrain boundary modified nanocomposite has a figure of merit ZT greaterthan 0.6 at temperatures up to 200° C.
 5. The thermoelectric material ofclaim 1, wherein said grain boundary modified nanocomposite has anelectrical conductivity at least 30 percent greater than an electricalconductivity for an analogous nanocomposite not having said zincantimony modified grain boundaries.
 6. The thermoelectric material ofclaim 5, wherein said grain boundary modified nanocomposite has athermal conductivity at least 35 percent less than a thermalconductivity for an analogous nanocomposite not having said zincantimony modified grain boundaries.
 7. The thermoelectric material ofclaim 6, wherein said grain boundary modified nanocomposite has a chargecarrier mobility at least 60 percent greater than a charge carriermobility for an analogous nanocomposite not having said zinc antimonymodified grain boundaries.
 8. The thermoelectric material of claim 7,wherein said grain boundary modified nanocomposite has a figure of meritZT at least 80 percent greater than a figure of merit ZT for ananalogous nanocomposite not having said zinc antimony modified grainboundaries.
 9. A thermoelectric material comprising: a grain boundarymodified nanocomposite having a plurality of bismuth antimony telluridematrix grains; a plurality of zinc oxide nanoparticles within saidplurality of bismuth antimony telluride matrix grains; and zinc antimonymodified grain boundaries between said plurality of bismuth antimonytelluride matrix grains; said grain boundary modified nanocompositehaving an electrical conductivity greater than 21,000 S/m and a thermalconductivity less than 0.6 at temperatures less than and equal to 200°C.
 10. The thermoelectric material of claim 9, wherein said grainboundary modified nanocomposite has a figure of merit ZT greater than1.0 at temperatures up to 100° C.
 11. A thermoelectric materialcomprising: a grain boundary modified nanocomposite having a pluralityof bismuth antimony telluride matrix grains; a plurality of zinc oxidenanoparticles within said plurality of bismuth antimony telluride matrixgrains; and zinc antimony modified grain boundaries between saidplurality of bismuth antimony telluride matrix grains; said grainboundary modified nanocomposite having an electrical conductivitygreater than 21,000 S/m and a thermal conductivity less than 0.6 attemperatures less than and equal to 200° C., said grain boundarymodified nanocomposite also having a figure of merit greater than 1.0 attemperatures up to 100° C.